1. Field of the Invention
The present invention relates generally to apparatus and methods for use in wavelength division multiplexed systems, and more particularly to the design and fabrication of optical add/drop filters for wavelength division multiplexed systems.
2. Discussion of Background Art
Fiber optic systems are used to transmit information for high performance communications systems and to interrogate sensors for a variety of applications, including monitoring chemicals and monitoring biomedical parameters. Generally, most such applications benefit from the ability to transmit multiple wavelengths down a single fiber optic cable for wavelength division multiplexed (WDM) systems. The amount of information transmitted down a single fiber is limited by technology and cost constraints on the transceiver units installed on the cable ends (the units that convert between electronic and optical signals). More wavelengths allow more information or more sensor discrimination, without the need for increasing the number of fiber cables. Avoiding an increase in cabling is advantageous because it keeps the cable connection size down (important for minimizing sensor size and for reducing space occupied by connectors on communications boards/cards), and because in some applications (notably network applications) it avoids the need to install more cable to an existing infrastructure, which in general is very expensive. Also, for high performance computer interconnects, the ability to transmit multiple wavelengths enables the destination of an in formation packet to b e simply encoded by mapping destination onto the optical wavelength. This in turn can enhance the performance of the interconnect significantly (see A. J. DeGroot et al, xe2x80x9cHigh performance parallel processors based on star coupled WDM optical interconnectsxe2x80x9d, Proc. 3d Intl Conf. on Massively Parallel Processing using Optical Interconnects, IEEE Computer Society, Oct. 1996).
For this reason, multi-wavelength filter technology has become very popular in telecommunications systems using SINGLE-MODE optical fiber, and a wide variety of products are currently on the market (eg: Ciena, Di-Con, OCA, etc.). In this environment, single mode fiber is used to enable hi-speed transmission over long distances (10 km and above). It""s use, however, substantially increases the cost of components (transceivers, wavelength filters) because it has a small optical spot size which results in very stringent alignment tolerances on the components. Single-mode (xe2x80x9cmonomodexe2x80x9d) fibers are also not attractive due to the high component cost of single-mode optical components, and lens approaches are less attractive because they add piece parts (cost) and bulk, leading to exceedingly bulky arrays.
For many other applications, however, it is preferable to use MULTI-MODE optical fiber, which has a spot size about 10xc3x97 larger than single mode fiber. This reduces component cost substantially (but limits transmission distance to about a kilometer at 1 Gbit/sec bandwidth). For these lower-cost systems, component cost is an important consideration, and it is important to achieve a low cost for the wavelength multiplexer. Single mode demultiplexers typically cost $1000/wavelength/fiber end, which is prohibitive. These components also will not work within a multimode system due to incompatibilities between single- and multi-mode fiber.
Parallel optical interconnects over multimode fiber (MMF) ribbon cable are emerging as a robust, high-performance data link technology (See, Y. -M. Wong et al., J. Lightwave Technol. LT-13, 995 (1995); M. Lebby et al., Proc. 1996 Electron. Components and Technol. Conf., p. 279 (1996); and, K. S. Giboney, Proc. SPIE Optoelectron. and Packaging IV (Feb. 1997)). This technology has primarily been implemented as single wavelength, point-to-point links, and can be significantly enhanced by wavelength division multiplexing (WDM) to increase both point-to-point bandwidth as well as create more complex interconnect topologies and routing approaches. The combination of byte-wide transmission for high channel bandwidth with WDM for interconnect routing is particularly attractive for ultrascale computing platforms (See, R.J. Deri et al., Proc. 3d Massively Parallel Proc. using Opt. Interconn., p. 62 (1996)). Research in this area suggests that WDM transceivers for point-to-point links can be realized (See, S. Y. Hu et al., in Proc. 1997 IEEE LEOS Annual Mtng., paper TuJ4 (1997); and, C. Chang-Hasnain, in Proc. 1997 IEEE LEOS Annual Mtng., paper WJ1 (1997)). Exploiting the potential richness of WDM networks, however, also requires a low-loss routing fabric which includes small footprint add/drop multiplexers. Low insertion loss is also critical for this technology because the transceivers exhibit link power budgets well below that of telecom WDM systems and because the multimode fiber cabling precludes the use of optical amplifiers. While high-performance filters can be realized for single-fiber applications (See, L. Aronson et al., presented at OFC ""97 ), achieving high-performance devices with ribbon cable is significantly more complicated. Complications arise from the MMF""s high NA=0.275 and large core (62.5 xcexcm), which render array collimation difficult, and the difficulty of maintaining good filter performance at the high angles of incidence needed to minimize loss in a 3-port (2-output) device.
In response to the concerns discussed above, what is needed is a design and fabrication method for optical add/drop filters in wavelength division multiplexed systems that overcomes the problems of the prior art.
The present invention is an optical add/drop filter for wavelength division multiplexed systems and methods for constructing same. The add/drop filter consists of a first ferrule having a first pre-formed opening for receiving a first optical fiber; an interference filter oriented to pass a first set of wavelengths along the first optical fiber and reflect a second set of wavelengths; and, a second ferrule having a second pre-formed opening for receiving the second optical fiber, and the reflected second set of wavelengths.
A first method for constructing the optical add/drop filter consists of the steps of forming a first set of openings in a first ferrule; forming a first set of guide pin openings in the first ferrule; inserting a first set of optical fibers into the first set of openings; dividing the first ferrule into a first ferrule portion and a second ferrule portion; forming an interference filter on the first ferrule portion; inserting guide pins through the first set of guide pin openings in the first ferrule portion and second ferrule portion to passively align the first set of optical fibers; attaching the second ferrule portion to the interference filter; removing material from the ferrule portions and interference filter such that light reflected from the interference filter from the first set of optical fibers is accessible; forming a second set of openings in a second ferrule; inserting a second set of optical fibers into the second set of openings; and positioning the second ferrule with respect to the first ferrule such that the second set of optical fibers receive the light reflected from the interference filter.
A second method for constructing the optical add/drop filter consists of the steps of forming a first set of openings in a first ferrule; inserting a first set of optical fibers into the first set of openings; cutting a slot into the first ferrule; removing predetermined portions of the ferrules and interference filter; inserting an interference filter into the slot such that light reflected from the interference filter from the first set of optical fibers is accessible; forming a second set of openings in a second ferrule; inserting a second set of optical fibers into the second set of openings; and positioning the second ferrule with respect to the first ferrule such that the second set of optical fibers receive the light reflected from the interference filter.
A third method for constructing the optical add/drop filter consists of the steps of forming guide pin openings in a first block; forming sets of openings in a first, second, and third ferrule; inserting sets of optical fibers into the sets of openings; forming guide pin openings in the first, second, and third ferrules; inserting guide pins through guide pin opening in the first block, and the first and second ferrules such that a first set of light wavelengths from the optical fibers in the first ferrule are passed into the optical fibers of the second ferrule; and inserting guide pins through guide pin opening in the first block, and the third ferrule such that a second set of light wavelengths from the optical fibers in the first ferrule are passed into the optical fibers of the third ferrule.
The apparatus and method of the present invention are particularly advantageous over the prior art because a means for realizing a low-cost wavelength multiplexer for an array of multimode optical fibers is taught. The array is useful because it can mate directly to high-performance transceivers designed to achieve high bandwidth by transmitting simultaneously over several multimode fibers within a multimode optical parallel optical interconnect (POI) fiber array. Alternatively, the array can be cut into individual elements after assembly, thereby providing a batch processing for single-fiber cables. Additionally, the present invention uses fiber ferrules with holes to contain optical fibers instead of surface-machined grooves or channels. This saves cost by reducing fiber-to-block mounting costs, avoids need of adding cover mounts to the fiber blocks, and enables use of injection molding for inexpensive block manufacture and ready adaptation to guide pins.
As a result, this invention demonstrates a simple fabrication approach for compact, high-performance WDM filters which are compatible with existing byte-wide transceivers. The filters are constructed from widely available ferrules to minimize alignment and connectorization costs, and exhibit low loss, sharp skirts, reasonable crosstalk suppression, and a negligible mode selective loss. Such a design is suitable for channel separations as small as 15-30 nm, and 5-10nm with some modification. The invention directly enables several WDM interconnects, including chordal rings. More significantly, in combination with recent advances in byte-wide WDM sources, the present invention will enable byte-wide WDM fabrics with appreciable source routing capability and high channel bandwidth.
The present invention is useful within communications systems where each optical wavelength carries different information. This includes multiplexers, demultiplexers, interconnects of computing nodes within a large parallel processing system, and local area networks between such a processing system and user workstations and/or archival storage. The present invention is also useful within interconnects of a high performance xe2x80x9cembeddedxe2x80x9d processing system for mobile platforms. Importantly, the present invention provides a means to enhance the bandwidth of local area networks without having to upgrade cable infrastructure.
These and other aspects of the invention will be recognized by those skilled in the art upon review of the detailed description, drawings, and claims set forth below.